Disc brakes can be used on all four wheels of a vehicle, or combined with disc brakes on the front wheels and drum brakes on the rear.
When the brake pedal is depressed, a push rod transfers the force through a brake booster to a hydraulic master cylinder. The master cylinder converts the force into hydraulic pressure, which is then transmitted via connecting pipes and hoses to one or more pistons at each brake caliper. The pistons operate on friction pads to provide a clamping force on a rotating flat disc that is attached to the wheel hub. This clamping tries to stop the rotation of the disc, and the wheel.
On non-driving wheels, the center of the brake disc or hub contains the wheel bearings. The hub can be part of the brake disc or a separate assembly between the wheel and hub with nuts or bolts. On driving wheels, the disc is mounted onto the driving axle and may be held in place by the wheel. On front wheel drive vehicles, it can be mounted on the front hub and wheel bearing assembly.
The brake caliper assembly is bolted to the vehicle axle housing or suspension. In most cases the brake is positioned as close as possible to the wheel, but there are exceptions. Some high-performance cars use inboard disc brakes on its rear wheels. The makers claim improved vehicle handling for this design because it reduces unsprung weight.
Applying brakes can absorb a lot of vehicle energy so friction between braking surfaces generates great heat. Brake parts withstand very high temperatures. Most of the friction area of a disc is exposed to air so cooling is far more rapid than for a drum brake. Unlike with drum brakes, brake fade is rare. Because of their shape, discs tend to throw off water. So after being driven through water, they operate almost immediately.
Disc brakes need much higher pressures to operate than drum brakes, so almost all disc brake systems need a power brake booster to help reduce the pedal forces that are needed from the driver. Because of the high forces needed to apply a disc brake, using it as a handbrake is less common. Some vehicles build a drum brake into the center of the rear disc to provide for park brake operation.
